TECHNIQUES IN PLANT VIROLOGYCIP Training Manual2.1 DETECTION/Symptomatology

Section 2.1.1Symptoms of PlantVirus Diseases

Symptoms are defined as the macroscopic or microscopic modificationsor variations (form, color, size) appearing in plants as a result ofpathogenic actions, nutrient deficiencies, adverse environmentalconditions, etc.

The study of viral disease symptoms requires careful periodicobservation of infected plants (indicator plants) to determine precisely thetype, intensity, and prevalence of any abnormality. The most evidentsymptoms are those appearing in the leaves, although some viruses maycause symptoms in the stems, fruits, roots, or other parts of the plant.

The most common symptom of viral diseases is slower developmentresulting in plant dwarfism. Almost all viral diseases reduce yield, and theeffect may range from severe and easily perceivable to negligible.

In most viral diseases of plants growing in the field, the virus affects thewhole plant (systemic infection). The symptoms are called "systemic." Inartificially inoculated plants, certain viruses cause small, usually necrotic,

P.V. • Sec 2.1.1 – 99 • Page 2 - INTERNATIONAL POTATO CENTER

lesions at the point of entry (local infection). The symptoms are called"local lesions."

Many viruses can infect certain hosts without the development of evidentsymptoms. They are called "latent viruses" and their hosts,"asymptomatic." In both cases, plants that usually develop symptomswhen infected by a virus may remain asymptomatic for a certain periodunder certain environmental conditions, such as high or low temperature.This is known as "masking."

Lastly, artificially virus-infected plants may show severe symptomsimmediately after inoculation, and eventually die. If the plant survives theinitial shock, the symptoms can be mild ("chronic symptoms") withsubsequent partial or total plant development. This is called "recovery."Alternatively, symptoms may gradually become more severe and result inslow or rapid plant degradation.

The symptoms of these types of viral infections are:

a) Local symptoms

Local symptoms usually appear on the leaves of inoculated plants. Theycan be discrete isometric lesions, which can be counted and, in somecases, used for quantitative studies. These symptoms are of noeconomic importance, but are very useful for diagnostic work.

Local symptoms include:

• Chlorotic spots

• Chlorotic rings

• Necrotic spots

• Necrotic rings

b) Systemic symptoms

Systemic symptoms appear in non-inoculated plant tissues. They are themost important symptoms because they can affect any part of the plant(flower, fruit, petiole, etc.).

Systemic symptoms include:

Mosaic. This symptom consists of alternating normal, green areas andchlorotic (light green) areas on the leaves. They appear irregularly and donot follow a fixed pattern. Occasionally, the symptoms start with the veinslosing their color (vein clearing). We can include mottling, vein banding,and chlorotic strips in mosaic-type symptoms.

Yellowing. This symptom consists of a partial or generalized chlorosis.Yellowing-type symptoms include chlorosis, calico, aucuba, veinyellowing, interveinal chlorosis, etc.

Necrosis. This symptom appears in systemic infections as apicalnecrosis, vein or stem necrosis, systemic necrotic rings, etc.

P.V. • Sec 2.1.1 – 99 • Page 3 - INTERNATIONAL POTATO CENTER

c) Growth abnormalities

These symptoms may appear in any of the plant parts. They include leafrolling, epinasty, blistering, sprout proliferation, rugosity, curling,coriaceous leaves, dwarfisms, etc.

Recommended LiteratureBos, L. 1978. Symptoms of virus diseases in plants. Centre for

Agricultural Publishing and Documentation. Wageningen. 225 pp.

Salazar, L.F. 1996. Potato viruses and their control. International PotatoCenter. Lima, Peru. 214 pp.

Walkey, D.G. 1985. Applied plant virology. 1st edition, London. 329 pp.